The whole purpose of a story is to see who wins in the end, the hero or the villain. That’s why it’s crucial for the hero and villain to have mutually conflicting goals. if one wins, the other loses. That forces them to fight to the death for their goals.
In “Die Hard,” the hero wants to save his wife by killing all the terrorists. Yet for that to happen, the villain (the head terrorist) must die and lose. Since the villain doesn’t want to die, he keeps fighting back against the hero until in the end, only one can win.
In “Star Wars,” Darth Vader wants to blow up the rebel base, but Luke Skywalker wants to save Princess Leia by blowing up the Death Star to stop it. If Luke wins, then Darth Vader loses. If Darth Vader wins, then Luke loses. The story ending must be as clear cut as that.
Even romantic comedies have an all-or-nothing conclusion. In “Crazy Rich Asians,” the hero wants to marry her boyfriend but the mother doesn’t want her to. That means only one of them can possibly win.
In “Moonstruck,” the hero wants to be with her fiancé’s brother instead of with her fiancé, but her fiancé wants to be with her. Only the hero or the villain can win.
What happens if you violate this principle so the hero and the villain don’t have mutually exclusive goals? Then it doesn’t really matter who wins and the story becomes far weaker and less emotionally satisfying.
In “The 5th Wave,” the hero’s goal is to rescue her little brother but the villain’s goal is to continue taking over the world as an alien from another galaxy. So it makes no difference if the hero rescues her little brother because it doesn’t affect the villain’s goal one bit. At the end of “The 5th Wave,” the hero succeeds and the villain is completely oblivious to the hero’s existence. This creates an ending where the hero and villain never battle, which is like watching a boxing match where the two boxers never fight. Dull and boring, which is just one (of many) reasons why “The 5th Wave” was such a disappointing movie.
When you think of an idea for a story, think of the conflict in the end. Make it big and make sure the hero’s goal and the villain’s goal are mutually exclusive. That sets up a final battle between the hero and villain so audiences will want to know who wins and how it turns out in the end.